The old rover was supposed to work for only 90 days, enough time to crawl two-thirds of a mile across the Martian desert. More than five years later, Spirit has put five miles on its odometer and is still rolling along — but getting mighty cranky.

The rover, one of two NASA vehicles operating on Mars, has a broken right wheel. It has dust on its solar panels. It’s operating at about 30 percent of normal power. Various sensors and software programs have gone screwy.

Then, on April 9, Spirit refused to wake up. The rover is designed to sleep at night, when there is no sunlight hitting the solar panels. But Spirit snoozed right through its wake-up call. It happened three times in succession. Finally, a backup timer got Spirit up and moving again after a 27-hour slumber.

John Callas, project manager for the Mars rovers at the Jet Propulsion Laboratory in Pasadena said he doesn’t have an explanation for what ailed Spirit. Nor can he explain why the rover repeatedly rebooted itself when engineers at the JPL tried to send it commands. The engineers found a fix — they relayed commands via a spacecraft that orbits Mars — but the incidents suggest that Spirit is getting erratic. Or maybe just old.

By any measure, Spirit and its sister rover, Opportunity, which is a good bit healthier, have been triumphs of the civilian space program. Spirit may yet operate for several more years, or it may be on its last legs. In any event, it is providing a tutorial on how even the most exquisitely designed machines eventually die.

“I don’t think anyone can tell you how these rovers will eventually end on Mars,” Callas said. “Will they gradually degrade until their mechanical functionality goes or will they have a catastrophic end, where something inside the rover breaks?”

When the rovers were designed, the presumption was that dust would blanket the solar panels within a few months of being on Mars and that the rovers would grind to a halt. Martian winds blew to the rescue, cleaning the panels periodically and letting the rovers extend their missions.

Spirit is now driving around a plateau called Home Plate in a valley known as the Inner Basin. Opportunity, on the other side of Mars, has left Victoria Crater and is rambling toward a large crater named Endeavour.

Day-to-day life on Mars can be rocky. The temperature can swing 150 degrees Fahrenheit between night and day because of the thin atmosphere. That heating and cooling cycle puts stress on metal.

“Metal parts and glass parts expand and contract as the temperature changes,” said John Casani, who has worked on robotic space missions for decades at the JPL. “If you take a piece of metal and keep bending it back and forth, pretty soon it’s going to break.”

Casani said spacecraft — the ones that stick to space and don’t try to land on a planet or some such feat — exist in a much more stable environment. Thus, the JPL is still getting data from the two Voyager craft launched in 1977. According to the JPL, the oldest functioning spacecraft is Voyager 2 (launched slightly earlier, strangely enough, than Voyager 1), which is zipping toward interstellar space far beyond Neptune’s orbit.

There’s not much to see out there, and, in any event, no operational camera to see it with, as the spacecraft’s batteries, powered by radioisotope decay, slowly become enfeebled. But a few instruments still function, and scientists have rebuffed efforts to shut down the Voyager program.

Spacecraft can also be sent to their deaths. Casani was the project manager on Galileo, a probe to the Jupiter system. When Galileo ran low on propellant, engineers knew that eventually it would lose attitude control and start tumbling. That raised the fear that Galileo would crash into one of Jupiter’s moons, which potentially harbor life and might be contaminated by stowaway Earth microbes. So Casani and his colleagues used the last bit of propellant to send Galileo into Jupiter, where it burned up in the thick atmosphere.

When things go wrong, scientists and engineers often have a workaround. Sometimes, they just get lucky, which is what happened when Spirit’s right front wheel broke three years ago.

The other five wheels on the rover were functional and were capable of dragging the broken wheel across the surface. The inoperative wheel, locked, gouged a trench as it went along. By examining that trench, Spirit was able to detect a certain kind of silica that offered evidence of ancient hot springs on Mars.

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